1. Field of the Invention
The invention generally relates to semiconductor manufacturing, and more particularly, to methods for increasing carbon and boron dopant concentrations in silicon and silicon germanium films that are epitaxially deposited.
2. Background of the Invention
In semiconductor manufacturing, methods have developed for making semiconductor devices, some of which are wafers based on silicon (xe2x80x9cSixe2x80x9d) or silicon germanium (xe2x80x9cSiGexe2x80x9d) films. Si films are widely used with many applications. For SiGe, an important use has been in electrical components known as heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d). Si films also are used for HBTs. For depositing Si and SiGe films, certain conventional epitaxial deposition techniques have been developed. U.S. Pat. Nos. 4,716,048; 4,726,963; 4,853,251; 5,018,479; 5,961,877; 5,986,287; 6,027,975; 6,064,081. Silicon and silicon germanium layers have been grown via ultra-high-vacuum chemical vapor deposition (xe2x80x9cUHV-CVDxe2x80x9d) low temperature epitaxy (xe2x80x9cLTExe2x80x9d) processes. After such growth of an Si or Sixe2x80x94Ge layer, collector, base and emitter doping regions are subsequently defined.
Those working with transistor technology have found high carbon (xe2x80x9cCxe2x80x9d) content in silicon and silicon germanium films to be important for maximizing performance of HBTs made from SiGe or Si films. In growing films, temperatures of less than 800xc2x0 C. generally are used, while in doping applications, lower temperatures generally are used, below 750xc2x0 C. There have been processes for providing high levels of carbon (i.e, for alloy manufacture) at high temperatures, greater than 800xc2x0 C., but these processes are unsuitable for semiconductor manufacture because carbon control cannot be controlled in the range of interest. When temperature is reduced, achieving carbon incorporation becomes a problem. Controllably incorporating carbon at ranges of less than 1% at low temperatures has eluded semiconductor manufacturers.
Although high levels of carbon doping have been desired in Si and SiGe films to obtain requisite device characteristics, conventional processes have not provided such high carbon levels, especially for SiGe films. When a germanium precursor source is started, the growth rate rapidly increases, providing too little time for introduction of desired dopants, such as carbon. The desired higher carbon levels have eluded SiGe film manufacturers, who generally have found that carbon incorporation in SiGe decreases significantly as the Ge content in the film increases. For certain conventional processes for making SiGe films, FIG. 1 summarizes data for carbon concentration as a function of the flow of the carbon source, with separate plots shown for various Ge content in the SiGe film. In FIG. 1, film growth is performed using Ge incorporation calibrations. The vertical displacement of the curves indicates carbon incorporation is strongly decreased for even slight Ge content. One implication of such data (i.e., FIG. 1) for conventional SiGe processes is that to achieve a desired level of carbon doping in SiGe (such as an SiGe film containing 15% Ge) may require a carbon flow 20 times (or more) the carbon flow used to produce the same carbon doping level in an Si film. Therefore, it has been conventionally thought that a process for doping SiGE films with a single carbon source would require a mass flow controller over a range exceeding two orders of magnitude, greater than what would be feasible. Thus, a single carbon source system appearing infeasible, the conventional method has been to use two ethylene source gases with different concentrations; one for doping Si and the other for doping SiGe. Such a method has its own problems, including undesirably requiring hardware additions to the LTE tool and also space for an additional gas source, making dual-source set-ups relatively costly and difficult to install and maintain. Also, with such a method, large pressure changes (normally undesirable for UHV-CVD growth) may occur for SiGe growth as more Ge is added.
The problem of increasing the carbon content in SiGe films has persisted and resisted solution. For example, it was thought that the high growth rates encountered during the SiGe growth with higher Ge fractions was responsible for the low levels of carbon incorporation, and thus, lowering the growth temperature was attempted. However, lowering the growth temperature by 15xc2x0 C. reduces the growth rate by a factor of ⅔ without significantly increasing carbon incorporation.
Thus, the conventional processes for doping Sixe2x80x94Ge and Si films do not necessarily produce films with desired characteristics. Even when processes can be constructed to produce certain desired characteristics, such processes are not necessarily simple or suited to large-scale manufacturing. There especially remains a need to improve carbon incorporation into SiGe films.
It therefore is an object of this invention to provide a method to improve incorporation of a dopant such as carbon or boron into Si and SiGe films, such as during production of a film by UHV-CVD. Advantageously, the present invention provides such desired higher levels of carbon in Si and SiGe film, using existing hardware and not requiring a higher-concentration carbon source or any additional mass flow controller. Another advantage of the present invention is that film growth may proceed at low pressures, and at temperatures of under 800xc2x0 C., preferably at 600xc2x0 C. or less. A further advantage of the invention is that by avoiding high temperatures, at which impurities such as oxygen necessarily would be introduced, introduction of impurities is correspondingly avoided. Additionally, the present invention advantageously provides pressure-balanced processes, in which pressure spikes during film growth may be avoided. In the methods and articles according to the invention, these benefits are provided without adversely affecting other growth parameters, such as the number of impurities, the number of defects, whether doping is spiked, controllability, manufacturability, etc.
In order to accomplish these and other objects of the invention, the present invention in a preferred embodiment provides a method of reducing film growth rate when growing a carbon- or boron-doped silicon film or silicon-germanium film, comprising carbon or boron-doping while supplying a silicon precursor and optionally a germanium precursor to a substrate, at reduced pressure of about 0.1 to 100 millitorr. The invention also provides a method of growing a film without sharp pressure transitions, by such a step.
In a preferred embodiment, the doping is at a temperature of less than 800xc2x0 C. In a particularly preferable embodiment, the doping is by a precursor supply that is a single source.
The inventive method in a preferred embodiment may include supplying germanium precursor to the substrate. In a particularly preferred embodiment, the film has a germanium content of 1 to 30% by weight.
In a preferred embodiment, the film has a dopant content of about 1xc3x971017 to 1xc3x971021/cm3. Another preferred embodiment is a silicon or silicon-germanium film doped with carbon or boron wherein the dopant profile is spiked. In a further preferred embodiment, the invention provides a transistor comprising a silicon-germanium-carbon layer with a carbon content of about 1xc3x971017 to 21/cm3.